(a) Field of the Invention
The present invention relates to a cruise control system and method for a vehicle.
(b) Description of the Related Art
With increased number of vehicles and congestion on the roadways, the need for more intelligently designed safety mechanisms is greater than ever before. In particular, efforts have been directed to creating improved anti-collision systems and methods. While adaptive cruise control (ACC) systems can help ensure the safety and comfort of the driver and his passengers even under complex road conditions, conventional cruise control systems cannot reliably avoid a collision or for warning against a collision.
In conventional ACC systems, once a running speed is set on a host vehicle, the ACC system on the host vehicle takes in certain load conditions and the running speed of the preceding vehicle as detected by sensors and, based on these parameters, controls the throttle actuator or brake actuator to maintain the host vehicle at a fixed predetermined running speed. The host vehicle is equipped with a distance detection unit mounted at the front which sends its signals to a computer that calculates a relative velocity and relative distance from the preceding distance. The computer also executes an algorithm that calculates and sets a time gap for determining a safety distance to leave between the host vehicle and the preceding vehicle based on the current running speed of the host vehicle. The safety distance is derived by multiplying the time gap by the current running speed of the host vehicle.
In this manner, if the relative distance and relative velocity indicate that the host vehicle is in danger of colliding with the preceding vehicle, the ACC system will apply the brakes and thereby control the vehicular distance to the preceding vehicle based on the predetermined time gap. When the threshold braking distance of the host vehicle as it corresponds to the current running speed thereof is less than the calculated safety distance, the ACC system operates the brake actuator or the throttle actuator until the distance from the preceding vehicle exceeds the predetermined safety distance.
Once the distance from the preceding vehicle exceeds the calculated safety distance due to the reduced running speed of the host vehicle, the ACC system will increase the running speed of the host vehicle to the original fixed running speed by recovering an engine torque.
One of the reasons for the imperfect anti-collision mechanisms of conventional ACC system is the manner by which the time gap is determined. As those of skill in the art will recognize, the time gap in conventional ACC systems cannot be readily modulated by a driver. Even in conventional systems that allow the driver to select the time gap to be one of three phases, Far, Med, and Close, the mechanism for control is such that it presents difficulty and danger for a driver attempting to adjust the time gap while driving. The simple fashion by which the time gap is determined is also a cause for concern. Assuming the predetermined time gaps of a host vehicle running at a speed of 100 km/h to be 2.0, 1.5, and 1.0 s, the distance from the preceding vehicle to be maintained would be 55 m, 42 m, and 28 m, respectively, using conventional ACC systems. The time gap is set with the assumption that the friction coefficient between each tire and a road is 1.0 and the maximum deceleration of the host vehicle is 9.8 m/s2. With these oversimplified conditions, the minimum braking distance for a host vehicle at a running speed of 100 km/h is 38 m and would appear well within the calculated safety distance of 55, 42, and 28 m.
Unfortunately, the assumption relied upon in conventional ACC system can be a fatal one as the friction coefficient between each tire and the road is not always 1.0. As shown in FIG. 6, the friction coefficient between each tire and the road can vary significantly depending on road/weather conditions. The friction coefficient between each tire and the road can also vary considerably due to the type of road surface, e.g. asphalt, concrete road, unpaved, etc.), thread design or degree of wearing on each tire. For example, the friction coefficient and the maximum deceleration for a vehicle on a concrete road with a new tire on a sunny day is about twice as large as the friction coefficient and the maximum deceleration for the same vehicle on an asphalt road with an old tire on a rainy day.
Conventional ACC systems fail to account for variations in the friction coefficient between each tire and the road when calculating the minimum safety distance. In addition, the maximum deceleration in the conventional ACC system is fixed to the value of 9.8 m/s2. As such, conventional ACC systems cannot be relied upon to provide the appropriate minimum safety distance and to avoid a collision between the host vehicle and the preceding vehicle in an emergency.
The above information is provided only to enhance understanding of the background of the invention and therefore it may contain information that does not form the prior art with respect to the present invention.